Well, if saying "hardest" you reference the most dificult race to play at lategame vs the other races I say P, on the other hand, T and Z are the most dificult to control, etcetera ;p
You cant just say no matter the mu, it depends to a lot of thing's like if the map its a big one like andromeda you will have a lot more resources to keep fighting than in Blue Storm for example.
It depends too on the tech tree ur opponent has, example versus mass upgraded carriers and corsair's u will have a really bad time as Zerg, but maybe easier with T ofc it changes to carriers and goons, but is ez to counter with mass goliats and tanks. And if u are terran you will have nightmares against Ultra/ling/defiler tech.
So i dont think there's a really hard race to play in late game than another. And hopefully this will not be another " My race pwn urs, my race is imba thread" T_T
sc is the most imba game ever, srsly. just look at the players. they are all of equal skill level and ... oh wait, no. they're not on equal skill level .. : >
P is easy to control >.<
i find terran hard to play late game if you are behind and short on exps >.<
Z is hard attacking right, you really need your units placed right in all attacks no matter if its early or late game >.<